So as to detect the presence of chemical substances, it is possible to use optical sensors implementing two types of phenomena:
a refraction index modification, PA1 a light absorption modification. PA1 either a direct intensity measurement (selective absorption measurement), PA1 or an interferometric measurement (real index measurement) with or without any simultaneous absorption measurement.
These modifications may either directly concern the substance to be measured (gas or liquid) or make use of a relay material which selectively adsorbs or absorbs the substance to be measured which modifies its optical characteristics.
Unfortunately, most of the time, these modifications are extremely slight but may be compensated in optics by a large length of interaction. In fact, the index variation induced by the substance to be measured .DELTA.n produces a phase variation .DELTA..phi.=2.pi./.DELTA. nL. If the minimum phase variation able to be detected in an interferometric diagram is .delta..phi., the minimum detectable variation .delta.n is .delta.n=.lambda..delta..phi./2.pi. L. If L is extremely large, .delta.n may be extremely small.
Similarly, the absorption .DELTA..alpha. directly influences the outgoing luminous intensity signal Is. If the minimum detectable relative variation is .delta.I/Is, (Is being the luminous intensity of the undisturbed system), the latter is proportional to .DELTA..alpha.L. The minimum detectable absorption variation is thus conversely proportional to L.
A French patent application FR-A-88 14433 filed on 4 November 1988 describes an integrated optical device used to measure the refraction index of a fluid including (a) a light guide of an effective index n1 of the guided mode formed on a substrate and comprising a guiding film intended to carry luminous beams and inserted between one lower film and one upper film having refraction indices smaller than that of the guiding film, (b) a zone for measuring the interaction of the light guide intended to be in contact with the fluid, the upper film at the level of the measuring zone having a thickness smaller than the penetration distance of the dying out wave of the guided luminous beam and, outside this interaction zone, a thickness larger than said penetration distance of this same dying out wave and formed, at least partly, in the light guide and comprising one optical reference circuit and one optical measuring circuit including the measuring zone so as to measure the phase jump introduced by a change of effective index n2 of the guided mode due to the fluid.
This device makes use of the fact that, in integrated optics, the effective index of a guided mode depends on all the parameters of the structure of the guide and in particular the thickness of the refraction indices of the various films constituting this guide. Also, the modification of one of these parameters, and in particular the refraction index of one of these films, modifies the effective index of the guided mode in question and thus locally introduces a phase modification of the light which may be detected by means of interferometry.
In conventional known types of optical devices, the need to implement a large length of interaction also results in having a large spatial requirement. In guided optics, these two constraints may be reconciled by the use of optical fibers.
However, it is then extremely difficult to have a reference independent of the step carried out concerning disturbances caused by various parastic effects, for example due to temperature or pressure variations or variations of the incoming light intensity.
The aim of the invention is to have a wave guide structure able to avoid these effects where it is desired to detect the presence of a fluid, gas or liquid, such as a concentration of methane lower than the explosivity limit (1%), and also detect the modification of the concentration of a solute in a liquid, for example pH time variations or an antigene-antibodies concentration variation in biomedical applications.